The mission of this program is to understand the fundamental mechanisms or sex determination in the model genetic organism Zea mays (maize), a species that produces unisexual flowers (called "florets") through the action of sex determination (SD) genes. In staminate (male) florets, pistils are eliminating by subepidermal cell death, a process that requires certain tasselseed genes (tsl, ts2 and ts5). Functional pistils in the plant escape tasselseed-mediated death through the action of the cell protection gene silkless1. In pistillate florets, stamen initials undergo a process of developmental arrest controlled by the phytohormone gibberellin. The specific objectives of this program are to characterize the processes of tasselseed-mediated pistil cell death, silkless-mediated pistil cell protection and gibberellin-mediated stamen cell arrest. In the course of these experiments, several hypotheses will be tested including: 1) that ts1 encodes a direct transcriptional regulator of ts2 and other pistil cell death genes; 2) that the ts2-encoded protein metabolizes a steroid or steroid-like compound to signal cell death in sub epidermal pistil cells; and 3) that the arrest of stamen cells involves a specific cell cycle arrest. To further extend the SD pathway, we propose a combination of genetic strategies to identify additional SD genes, and biochemical approaches to identify TS2-, TS I - and SKI -interactive proteins. A battery of genetic tests will be used to characterize new tasselseed mutations to identify additional genes in the SD pathway. The genomes of several unisexual and bisexual grasses, such as Tripsacum and rice, as well as bisexual dicotyledonous plants, such as Arabidopsis, contain conserved homologs of the maize SD gene tasselseed 2. The conserved role(s) of ts2 homologs will be investigated in Arabidopsis using reverse genetics approach to recover and characterize mutations in ten tasselseed 2-like (TSL) genes. Cell death, or apoptosis, is essential for many processes in humans and its misregulation is implicated in many human diseases including cancer, infectious diseases like or Hepatitis B or AIDS, autoimmune diseases and neurodegenerative diseases like Alzheimer's. The study of the maize sex determination pathway, and specifically pistil cell death and protection, will provide an independent genetic approach in a non-animal system to understanding why and how cells are programmed to die or how cells are protected from death. Moreover, the study of maize ts2 and Arabidopsis TSL genes provides a powerful genetic approach to understand the biological function and regulation of steroid dehydrogenase/reductase (SDR) genes. SDRS have been implicated in the many hormone-related cancers that share a common steroid-based mechanism of tumorigenesis, namely breast, ovarian, endometrium, testicular, prostate, thyroid and osteosarcoma. Hence, understanding the processes of tasselseed-mediated cell death, silkless-mediated cell protection and gibberellin-mediated cell arrest will be of fundamental importance in understanding many health-related processes.